1. Technical Field
The present invention relates to a semiconductor device having a recess or protrusion provided on a substrate and, more particularly, to a semiconductor light emitting device that improves its external efficiency by having the recess or protrusion provided on the substrate.
2. Background Art
A semiconductor device, for example, a light emitting diode (LED) essentially comprises an n-type semiconductor layer, an active layer and a p-type semiconductor layer laminated on a substrate in this order. Electrodes are formed on the p-type semiconductor layer and on the n-type semiconductor layer. Light is generated in a light emitting region of the active layer through recombination of holes injected from the p-type semiconductor layer and electrons injected from the n-type semiconductor layer. The light is extracted to outside from a surface whereon the electrodes are formed, or from a back surface of the substrate where a semiconductor layer is not formed.
A light emitting diode having such a structure as described above requires control of this laminated semiconductor structure at an atomic-layer level, and for this purpose, it is a common practice to process a substrate surface to a mirror finish. The semiconductor layers and the electrodes are formed parallel to each other on the substrate, and a light-propagating portion is formed so that light propagates through a semiconductor portion. The light-propagating portion is formed so that a semiconductor layer having a high refractive index is sandwiched between the substrate and an electrode, both of which have lower refractive indices. A waveguide is interposed between a p-type semiconductor layer-electrode interface and a substrate-electrode interface.
When light generated in the semiconductor layer enters with an angle of incidence not smaller than a critical angle with respect to interfaces with the electrodes or an interface with the substrate, the light laterally propagates while repeating total reflection within the semiconductor layer. As a result, light is trapped within the waveguide and cannot be extracted to outside efficiently. In addition, part of light is absorbed while laterally propagating with repeated total reflection within the semiconductor layer, resulting in lower external quantum efficiency.
A method of roughing a top surface and side faces of a light emitting diode chip has been proposed, but this causes damage on the semiconductor layer and results in cracks and/or other trouble. This leads to partial breakage of a p-n junction and reduction in an effective light emitting region.
Japanese Unexamined Patent Publication (Kokai) No. 11-274568 proposes a method of increasing external quantum efficiency by creating a recess or protrusion on a surface of a substrate so that light generated in a light emitting region is scattered. This method employs a mechanical process or etching for randomly roughing a surface of a sapphire substrate of a GaN-based LED comprising the sapphire substrate, an n-type GaN layer, a p-type GaN layer and a transparent electrode that are laminated in this order. This process is supposed to scatter light incident on the sapphire substrate so as to increase the external quantum efficiency.